Method of making electroluminescent layers



Dec. 15, 1959 J. M. N. HANLET 2,917,442

METHOD OF MAKING ELECTROLUMINESCENT LAYERS Filed D60. 28, 1956Translucenf e/ec trade Semi -die/eefric layer E lee/rode layer Baseplate INVENTOR JACQUES M.N.HANLET I ATTORNEY United States Patent METHODOF MAKING ELECTROLUMINESCENT LAYERS Jacques Marie Noel Hanlet, Paris,France, assignor to Societe dElectronique ct dAutomatisme, Courbevoie,France Application December 28, 1956, Serial No. 631,224

Claims priority, application France December 30', 1955 9 Claims. (Cl.204-192) The present invention relates to electroluminescentcompositions for use in elect'ro-optical converters and the like whereinthey are included as semi-dielectric layers or films inserted between apair of electrically conducting armatures, one of which at least istranslucent and across which is applied an alternating electro-motiveforce. It is well-known that, in such compositions, two distinctmaterials at least are included, one of which emits electrons, whensubmitted to the said electric force, and thus acts as an activator forthe other one of these materials which emits photons in response to thesaid emission of electrons from the first one.

It more particularly relates to such electroluminescent compositionswhich may be said to be' of an alloxide kind in that the one and theother of the said two materials, activated and activating, areconstituted by oxides of metals and/or metalloids in a weight ratio notexceeding 1% of the activating material with respect to the activatedmaterial therein. Such compositions have been fully described in mycopending application No. 631,226 wherein, further to a non limitativebut preferred listing of the said oxides, viz. zinc, calcium, barium andcadmium oxides for the activated material and copper, lead, manganese,silver, thallium, selenium and germanium oxides for the activatingmaterial, it has been shown that an excess of free molecules of oxygen,not chemically combined with the said oxides, and introduced in thesaidcompositions, was an important factor as concerns the efficiencythereof.

An object of the invention is to provide a new and improved method ofmaking pellicular layers or films of such all-oxide electroluminescentcompositions upon electrically conducting armatures or base platestherefor.

Another object of the invention is to provide a method such that thesaid layers or films are obtained as monocrystalline coatings ofcomplexes of their component oxides.

A further object of the invention is to so provide this method so thatthe products resulting therefrom are totally independent of the rawmaterials from which they are made.

A further object of the invention is to so provide this method that thesaid layers or films are coated upon their armatures or base plateswithout any introduction of adhesive materials into the saidcompositions, thereby avoiding any introduction of dielectric losses inthe finally obtained product other than those inhering in thecomposition itself. 7

According to the invention, a method of making an all-oxideelectroluminescent coating upon an armature .particles and effecting thecrystallisation of the oxide "ice 2 layer resulting from the saidtransfer and the said oxidization.

According to the invention, further, such a method also includes a stepfor introducing an excess of non-metallic free molecules, e.g. oxygenmolecules in the said crystalline layer.

Preferably but not necessarily, the steps of transferring part of thealloy to the said armature, oxidizing the transferred particles andeffecting the crystallization of the formed coating are, partly atleast, concomitant.

In the accompanying drawing Figure 1' shows apparatus for carrying outone step of the process; Figure 2 shows apparatus for carrying outanother step; and Figure 3 shows the completed electroluminescentdevice.

The invention may be put into practice according tothe followingdisclosure:

In a vessel 10 the atmosphere of which may be conditioned at will, foreffecting the different steps leading to the final result sought herein,is first arranged a plate 11 of a material the required qualities ofwhich will be hereinbelow determined, and for instance a plate ofmagnesium. The surface of this plate will be of dimensions which aresubstantially equal to those of the electroluminescent layer to beproduced. Facing the said plate, which will later act as theintermediary alloysupporting plate, several crucibles 12, for instancesilica crucibles, are arranged in an isotropic pattern with respect tothis plate, i.e'. for instance, the said crucibles are placedat thecorners of a square support facing the useful face of the saidintermediary plate. The opening of each one of the said crucibles is sosmall with respect to the surface of the said plate that it may beconsidered as' a point source of material. In the said crucibles areplaced small loads of the metal or metalloid powders of which the oxidesmust be formed in the conc'erned electroluminescent composition to beobtained. The said crucibles may be heatedby heaters 13 and the saidintermediary base plate may be cooled, forinstance by means of acirculation of a refrigerating fluid through a cooling jacket 14 appliedto the rear face of plate 11 opposite to the face thereof exposed to thecrucible openings. Jacket 14 and plate 11 are supported upon supplyconduits 14a and 14b passing through base B- of the jarlll. v The vessel10 is evacuated by pump P up to a vacuum of the order of 10* and thecrucibles are heated up to the temperature of thermal evaporation of thecontents thereof. Obviously the evaporated products will condense uponthe cool surface of the said intermediary base plate. As two distinctmaterials at least are placed within the crucibles, the coating whichdevelops upon the cool surface will be an alloy of the evaporatedmaterials. Due to the even distribution of the crucibles, this alloycoating will be of uniformly increasing thickness and in the said alloythe component materials will present a uniform distribution dependingupon the opening surfaces of the crucibles wherein they are containedand/or their distribution amongstv the said crucibles. The time intervalduring which the evaporation process is maintained obviously dependsupon the required thickness' of the alloy and the said thickness must besuitable for the further steps of preparation of the finalelectroluminescent layer.

In the said alloy coating, however, the weight proportions of thedifferent components thereof will be controlled by the temperature ofthe crucibles. It is known that the weight or quantity of a metal or ametalloid which is evaporated under vacuo from a thermal process duringa fixed time interval is fundamentally dependent upon the temperatureapplied to this material for such an evaporation. Illustratively, forinstance, copper will -.intermediary base plate.

. a 3 be evaporated at a rate of 10* gram per second per cm. of surfaceof the copper source, when copper is heated at 1000" C., but the rate ofevaporation will be of 4X 10- gram per' second 'per square centimeterwhen the copper is heated up to 1100 C. The same will be true for anyand all of the materials which have been listed hereinabove as possiblecomponents of electroluminescent compositions according to the presentinvention. It is consequently apparent that, for a single temperature ofthe crucibles, the proportions of the var- IOllS component materials canbe accurately controlled in the resulting alloy from a suitableprovision of proportions of the relative opening surfaces of thecrucibles wherein they are distributed. When, for instance, thesecomponent materials are copper and zinc, an alloy containing 0.8% ofcopper and the remaining percentage of zinc will be obtained whenheating the crucibles at a temperature of 1100 C. The overall surface ofthe Zn-source is provided equal to onehalf of the overall surface of theCu-source since, at thesaid temperature, the rate of evaporation of thezinc is equal to milligrams per second per square centimeter.

It must be noticed that, instead of putting into the crucibles, purifiedmetallic powders, it will sufiice to place into the said cruciblesunpurified parcels of the sa1d metals as the phenomenon of selectivethermal evaporation will automatically eliminate any impurities in .thesaid parcels.

The evaporation process useful for the formation of anelectroluminescent film only lasts several seconds after ctheevaporation temperature has been reached, after which the .vessel iscooled.

The material of the intermediary base plate upon which is formed thesaid alloy must obviously be such that it rhodium surface 2 prepared topresent the so-called optical polish.

An atmosphere of pure oxygen is now admitted to vessel 10 throughconduit 16 and is established within the vessel under a pressure whichwill be maintained at a constant value during the second part of theoperative process. Thereafter a DC. potential difference is appliedbetween the alloy-bearing plate and the armature plate, with thepositive pole of the DC. source to thesaid armature plate. The value ofthe said potential difference is'such that the combination of saidplates acts as an ionic discharge device wherein the ionic current willhave a value from 70 to 100 milliamperes per square decimeter of theopposite surfaces. Such a density of the said ionic current is notcritical but preferred as avoiding an overheating of the electrodes andconsequently avoiding parasitic chemical reactions such as a diffusionof zinc or copper (when the said alloy consists of zinc and copper)within the material of the This material must then be so chosen as toavoid such a diffusion effect during this step of operation.

In this ionic discharge or cathodic projection process, the alloycoating acts as a cathode which emits electrons and the gaseousatmosphere ensures the formation of ions which are directed upon thesaid cathode and impact thereon, due to the acceleration of the gasmolecules under the action of the electrostatic field establishedbetween the plates. For a suitable relative adjustment of the distancebetween these plates, the gas pressure within the thickness of onemicron. electroluminescent layer will, for instance, be of the orderconditioning for the crystallisation thereof.

vessel and the value of the potential difference between the said platesthe potential gradient in the neighborhood of the cathode is made veryhigh and consequently the impacts of ions thereupon extract metalparticles therefrom. These freed particles are attracted toward theanode plate and consequently the armature is progressively coated with alayer of a complex of oxides as the said particles are oxidized as soonas extracted from the alloy or even sooner due to the oxygen atmosphereestablished within the said vessel.

As a merely illustrative example, the spacing between the plates may beof 9 millimetres for a potential difference of the order from 700 to 800volts D.C.

For the above example of a mixture of zinc and copper oxides, thepercentage of components making up the metallic alloy on theintermediary base plate will not be changed in the thin layer resultingfrom the transfer and oxidation step since the zinc will be extractedfrom the alloy at a rate of 340 milligrams per ampere hour and thecopper at a rate of 300 milligrams per amperehour. For other components,it shall be necessary to provide in the alloy a ratio of the componentsthereof different from the ratio required between the oxides in thefinally obtained layer. Such a condition will be easily met as thevalues of the extraction coefficient, measured in electron-volts, arewell-known for the various metals and metalloids concerned herein. When,for instance, silver is substituted for copper in the alloy, all theremaining conditions of operation remaining the same as herein abovedefined, the alloy which is formed on the intermediary base plate 11during the first step will contain a quantity of silver equal to aboutone-half only of the corresponding quantity of copper since the rate oftransfer of silver is of about 740 milligrams per ampere hour under thesame conditions.

Obviously the material of the intermediary base plate must be of a rateof transfer substantially lower than the rates of transfer of thecomponent materials in the alloy. In this respect also the use ofmagnesium for constituting the said base plate is quite justified, therate of transfer of magnesium under the stated conditions being of theorder of 9 milligrams per ampere hour.

The transfer operation is maintained as long as useful for thedeposition upon the armature plate of the required thickness of thelayer of oxides. It is for instance necessary to ensure the transfer of0.4 milligram of the alloy consisting of zinc and copper per squarecentimeter orthe armature plate for establishing a coating having a Asuitable thickness of the of 10 microns. Consequently, and with adensity of the current of milliamperes, the transfer operation will lastabout seven minutes.

The advantage of making the plate armature with a rhodiated surfacemachined up to an optical polish may be explained as follows: theresistance to extraction of particles of this armature plate must behigh; the melting temperature thereof must also be high; the opticalpolish of the surface is useful for a fair efiiciency of the converter.Rhodium appears to be satisfactory for these three conditions, butnickel could also be used though its melting temperature and opticalreflectivity coeflicients are lower than those of rhodium.

The oxide coating must then be submitted to a thermal This step may .beoperated through the means of a high frequency heater 17 surrounding thearmature plate or that part of the vessel which surrounds the locationof the said armature plate. Special advantage is obtained when the heatprocess for crystallization is made during the transfer and oxidizingoperation proper, as in such a case the oxide layer will be obtained asa single crystal of a complex of 'n'dsubstantial risk of reaching a toohigh temperature of the armature plate.

As a specific embodimentof the above described process a magnesium baseplate is placed in an evacuated bell jar. The bell jar rests on asuitable support plate. Placed on the plate support are four silicacrucibles arranged one at each corner of said support plate with regardto the magnesium plate facing them. Each crucible contains an intimatemixture of powdered copper and zinc with the quantity of zinc equal toone half the quantity of copper. Each crucible is provided with a pinpoint opening to provide a point source of evaporated metal. Themagnesium base plate is cooled by circulating a refrigerant fluid on theface opposite that exposed to the crucible openings. The crucibles areheated to 1100 C. to produce a layer of alloy containing 0.8% copper and99.2% Zinc on the base plate.

To carry out the next step of the process the bell jar is cooled and anarmature plate is placed therein parallel to the base plate and 9millimeters therefrom. The cruoibles are removed. The armature platepossesses a rhodiated surface machined to an optical polish. Anatrriosphere of pure oxygen is maintained within the jar. A DC.potential difference is applied between the alloybearing plate andthearmature plate with the positive pole of the D.C.- source running tosaid armature plate. A current density of 100 milliamperes is maintainedfor about seven minutes to deposit an electroluminescent mixed oxidelayer of about 1 0 microns thickness on the armature plate by cathodicprojection. To obtain crystalliz'ation of the oxide layer, the armatureplate is heated by means of a high frequency heater surrounding the jarat the level of said armature plate. Heating is carried out during theentire seven minute transfer period, so as to impart a temperature tothe rhodiated surface less than its melting point (about 1985 C.).

After the formation of the electroluminescent layer and the cooling ofthe armature plate, the other armature of the electro-opticalelectroluminescent converter may be made within the same vessel as usedheretofore. A translucent armature of a metal such as silver or aluminummay, for instance, be formed over the electroluminescent oxide layerfrom an evaporation in vacuo of the said metal. This step however doesnot form part of the invention and the second armature of the convertercan be made by any suitable process other than the above and appliedelsewhere.

Referring back to the transfer operation proper, it may be stated that,when the particles of the alloy are extracted therefrom by the impactsof ions and are brought to the armature plate, the energies of theelectrons of the internal orbits of the said molecules, viz. of theorbits which are the nearer to the nuclei of their atoms, are notchanged at all Whereas the energies of the electrons of the externalorbits thereof, viz. the energies of the socalled valence electrons, aresubstantially modified. Under the influence of the intermolecularfields, the crystallized body behaves as a single homogeneous electronicsystem. The identical energy levels of the distinct molecules thereofare converted into as many levels of energy within the said body. Forall the molecules therein, the quantic levels cannot comprise more thantwo electrons at most, and only the valence electrons act for thedetermination of the conductibility of the said body since only the saidelectrons will participate in the transfers of charges which will definethe electric current through the said body. Otherwise said, in themonocrystalline layer and when an AC. electromotive force is appliedthereto, an electron can only pass from an energy level to another onewhen the latter presents at most a single electron; when this secondenergy level is previously occupied by two electrons, such a passagecould not occur. Outside this normal zone which is more or less filledwith electrons, there exists within the body another zone ofconductibility which comprises as many quantic levels as .the saidnormal zone. The energy difference between these two zones is adequatelysmall so that electrons are brought from the normal to theconductibility zone from the action of the driving field. Such atransfer of electrons would only necessitate a very small work andconsequently the exchange of energy between the said zones would bequite small. It appears of advantage to provide an intermediate energylevelbetween the said zones and this may be obtained by producing withinthe crystalline layer either an excess or a lackof non-metallic (ornon-metalloid) atoms. When for instance an excess of atoms of oxygen isestablished within the copper oxide, the electrical conductivity of thesaid oxide will greatly increase: e.g. the introduction of 1% of oxygenin the copper oxide will multiply the conductibility thereof by acoeflicient equal to 10 at a normal temperature. On the other hand, anexcess of oxygen within the zinc oxide will appreciably reduce theconductibility thereof. .A lack of oxygen atoms in the same oxides willproduce corresponding opposite effects.

When the transfer and the oxidizing steps are simultaneously made, asdescribed above, the transfer being then made within an atmosphere ofpure oxygen, an excess of oxygen will be automatically obtained in theoxide layer, which is the wanted result. Of course it will be theadjustment of the pressure of the said oxygen atmosphere which will bethe controlling factor of the oxygen percentage introduced into theoxide layer. But the heating process for the crystallisation of the saidlayer will also act upon the said introduction: the number of oxygenmolecules trapped within the oxide layer will at first be quiteimportant but, when the baking is made, a certain balance is establishedbecause the heat gives to the atoms of the one and/or the other of thecomponent materials, e.g. zinc and copper, the possibility of balancingout part of these excedentary molecules from the crystal. The adjustmentof the pressure of the atmosphere of oxygen and of the temperature ofcrystallisation of the oxides can be made such that a definite result isobtained as concerns the presence in the layer of a required percentageof free'molecules of oxygen.

As said the crystallisation may be made after the oxide layer has beencompletely obtained. This does not change the final result of the above.

Further, the transfer operation need not be permanently made within anatmosphere of oxygen and this operation may be made with a filling of aninert gas such as argon or hydrogen. The oxidizing and crystallisingsteps would be postponed until after the transfer is completed.

Another manner of proceeding to the transfer operation is to introducefirst an oxygen atmosphere within the vessel and apply the said DCpotential difference across the intermediary and the armature plate; toensure an oxidation of the surface of the alloy upon the transfer plate;and then to cut the admission of oxygen and substitute thereto an inertgas atmosphere of higher atomic weight such as argon for instance. Theuse of a high atomic weight gas may be considered as being of advantagefor the transfer proper as it leads to a shorter time interval for thesaid operation because of the more efficient operation thereof in theextraction of oxide particles from the superficially oxidised alloy,but, of course, the processing or control is more elaborated due to thechange of atmosphere during the formation of the oxide layer onto itssupporting armature plate. The crystallisation may be made during such atransfer of particles of oxides or it may be postponed at will. In thefirst case, the crystal must thereafter be submitted to a diffusionprocess for the introduction therein of the said excess of molecules ofoxygen. In the second case, the crystallisation and the introduction ofthe said excess of oxygen molecules into the oxide layer may besimultaneously made by heating the layer within an atmosphere of oxygen.In both cases, the pressure of the atmosphere of oxygen must be higherthan the pressure adjusted for base plate 4 is formed comprises,

. oxide coating, and

7 been herein above described. A plate of aluminum may be used, as afurther substitute, as an intermediary base plate instead of magnesium.

The electroluminescent device produced according to the invention isshown in Figure 3, whereinthe thin semi-dielectric layer is shown at 1,between two armatures 2 and 3. The armature 2 previously deposited on ofeither rhodium or nickel, and translucent armature3 is formed of eithersilver or aluminum. In operation, an alternating voltage is appliedacross armatures 2 and 3 from source 5.

Having thus described and ascertained my invention,

I claim:

1. A method of making an electroluminescent coating of at least oneactivating oxide and one activated oxide upon an armature of anelectro-optical converter which thermally evaporating within anevacuated vessel a mixture of materials selected from the classconsisting of the metals and metalloids which are to make up said oxidecoating, condensing the evaporated materials upon the surface of anintermediate base plate to form an alloy, transferring particles of saidalloy to an armature by cathodic projection wherein the alloy serves asa cathode and the armature serves as an anode plate, oxidizing thetransferred particles to produce an heating said oxide coating to effectcrystallization thereof, the quantity of activating oxide 1 in thecoating being maintained at not more than 1% of the weight of theactivated oxide.

2. A method according to claim 1 wherein the quantity of activatingoxide is maintained at not more than 1% of the weight of the activatedoxide by proportioning the surface of evaporation of the materials withrespect to their respective temperatures of evaporation.

3. A method according to claim 1 and wherein the proportioning of thecomponent materials in the said 'alloy are determined of the transferredcoating upon the saidarmature.

4. The process of claim 1 wherein excess uncombined oxygen is introduceditno the oxide layer'by conducting the ionic discharge transfer step inan atmosphereof pure oxygen.

5. The process of claim 1 wherein excess uncombined oxygen is introducedinto the oxide layer by conducting the heating of the oxide coatingwithin an atmosphere of oxygen.

6. The process of claim 1 wherein the components of the alloy are zincand copper, and the intermediate base plate is of magnesium.

7. A method according to claim 1 wherein thecathodic projection isconducted in the absence of oxygen, and wherein the heating of thecoating on the armature is conducted within an atmosphere of oxygen toproduce a difiiusion of free oxygen molecules into said coating.

8. A process according to claim 1 wherein said heating of the oxidecoating is accomplished by high frequency heating of the armature.

9. A method according to claim 8 and wherein the armature is heated by ahigh frequency process during the concomitant operations of transfer andoxidation of the particles of the alloy onto the said armature.

References Cited in the file of this patent UNITED STATES PATENTS1,376,604 Case May 3, 1921 1,624,071 Richardson Apr. 12, 1927 1,982,774Winkler et a1 Dec. 4, 1934 2,066,081 Ives Dec. 29, 1936 2,112,975Penning et a1 Apr. 5, 1938 2,151,457 Williams Mar. 21, 1939 2,189,580

Hewlett Feb. 6, 1940 with respect to, theircathodic pro-

1. A METHOD OF MAKING AN ELECTROLUMINESCENT COATING OF AT LEAST ONEACTIVATING OXIDE AND ONE ACTIVATED OXIDE UPON AN ARMATURE OF ANELECTRO-OPTICAL CONVERTER WHICH COMPRISES, THERMALLY EVAPORATING WITHINAN EVACUATED VESSEL A MIXTURE OF MATERIALS SELECTED FROM THE CLASSCONSISTING OF THE METALS AND METALLOIDS WHICH ARE TO MAKE UP SAID OXIDECOATING, CONDENSING THE EVAPORATED MATERIALS UPON THE SURFACE OF ANINTERMEDIATE BASE PLATE TO FORM AN ALLOY, TRANSFERRING PARTICLES OF SAIDALLOY TO AN ARMATURE BY CATHODIC PROJECTION WHEREIN THE ALLOY SERVES ASA CATHODE AND THE ARMATURE SERVES AS AN ANODE PLATE, OXIDIZING THETRANSFERRED PARTICLES TO PRODUCE AN OXIDE COATING, AND HEATING SAIDOXIDE COATING TO EFFECT CRYSTALLIZATION THEREOF, THE QUANTITY OFACTIVATING OXIDE IN THE COATING BEING MAINTAINED AT NOT MORE THAN 1% OFTHE WEIGHT OF THE ACTIVATED OXIDE.